Section 2
Section 2
Cnidarians
Focus
Two Body Forms
Objectives
Overview
Before beginning this section
review with your students the
objectives listed in the Student
Edition. This section discusses the
characteristics of cnidarians, as well
as examples of cnidarians and their
basic body forms. Students will learn
about the three different classes of
cnidarians and how cnidarians
reproduce.
Bellringer
As the fragile bell of a jellyfish moves rhythmically through the
water or the flowerlike sea anemone sways gently in the ocean currents, it’s easy to be caught up in the mystery and beauty of these
animals. But don’t be deceived by their allure, for jellyfish and sea
anemones are carnivores that can inflict a vicious sting. Along with
hydras and corals, these animals belong to the phylum Cnidaria (nih
DAIR ee uh). Cnidarians have two basic body forms, as shown in
Figure 6, and both show radial symmetry. Medusa (muh DOO suh)
forms are free-floating, jellylike, and often umbrella-shaped. Polyp
(PAHL ihp) forms are tubelike and are usually attached to a rock or
some other object. A fringe of tentacles surrounds the mouth,
located at the free end of the body. Many cnidarians exist only as
medusas, while others exist only as polyps. Still others alternate
between these two phases during the course of their life cycle.
The cnidarian body has two layers of cells, as illustrated by the
hydra in Figure 7. The outer layer derives from ectoderm, and the
inner layer derives from endoderm. As in the sponge, there is a middle layer of mesoglea. But cnidarians differ from sponges in that
cnidarians’ cells are arranged into tissues.
● Describe the two cnidarian
body forms.
8C TAKS 2
● Summarize how cnidocytes
function.
10A TAKS 2
● Summarize the life cycle of
Obelia.
8B
● Compare three classes of
cnidarians.
8C TAKS 2
● Compare asexual and
sexual reproduction
10A TAKS 2
in cnidarians.
Key Terms
Encourage students to examine
Figure 6, which shows the difference
between medusas and polyps. Have
them write a short song, poem, or
limerick to help them remember
how these two forms are different.
Have volunteers recite their work to
the class. TAKS 2 Bio 6D (grade 10 only)
medusa
polyp
cnidocyte
nematocyst
basal disk
planula
Figure 6 Cnidarian body forms
The two body forms of cnidarians—medusa and polyp—consist of the same
body parts arranged differently.
Medusa
Motivate
Identifying
Preconceptions
Mouth
Ectoderm
GENERAL
Show students photographs of jellyfish, hydra, sea anemones, and
corals. Ask students about their
own experiences with these cnidarians. Have they ever seen a jellyfish,
a Portuguese man-of-war, a coral,
or a sea anemone? Have they ever
touched a cnidarian? What did it
feel like? Where have they seen
cnidarians-in fresh water or salt
water? Take this opportunity to
explain that although most cnidarians are marine. Some, such as
hydras, live in fresh water.
LS Intrapersonal
www.scilinks.org
Topic: Cnidarians
Keyword: HX4048
Mesoglea
Gastrovascular
cavity
Endoderm
Tentacle
Mouth
Basal disk
Evolutionary Milestone
2 Tissues
The cnidarian body plan is more complex than that of a sponge—it
contains specialized tissues that carry out particular functions. The
tissues, however, are not organized into organs.
622
Chapter Resource File
pp. 622–623
Student Edition
TAKS Obj 1 Bio/IPC 2A, 2C
TAKS Obj 2 Bio 6D
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TEKS Bio 6D, 7B, 8B, 8C, 10A
TEKS Bio/IPC 2A, 2C
Teacher Edition
TAKS Obj 1 Bio/IPC 2A, 2C
TAKS Obj 2 Bio 6D, 8C, 10A
TEKS Bio 6D, 8C, 10A
TEKS Bio/IPC 2A, 2C
622
Polyp
Tentacle
• Lesson Plan GENERAL
• Directed Reading
• Active Reading GENERAL
• Data Sheet for Math Lab
GENERAL
Transparencies
TT Bellringer
TT Two Cnidarian Body Forms
Chapter 28 • Simple Invertebrates
Planner CD-ROM
• Reading Organizers
• Reading Strategies
• Basic Skills Worksheet Microscope
Magnification
Cnidocytes
Flexible fingerlike tentacles surround the opening to
the gastrovascular cavity of cnidarians. Located on
the tentacles are stinging cells called cnidocytes
(NIH doh siets), also shown in Figure 7. Cnidocytes
are the distinguishing characteristic of the animals
in the phylum Cnidaria. Within each cnidocyte is a
small barbed harpoon called a nematocyst (nehm
AAH toh sihst). Nematocysts are used for defense
and to spear prey. Some nematocysts contain deadly
toxins, while others contain chemicals that stun but
do not kill. When triggered, the nematocyst
explodes forcefully and sinks into the cnidarian’s
prey. The captured prey is then pushed into the
cnidarian’s gastrovascular cavity by the tentacles.
Figure 7 Cnidarian body plan
Like all cnidarians, this hydra is composed of
tissues derived from endoderm and ectoderm.
READING
SKILL
BUILDER
Nematocyst
(discharged)
Interactive Reading Assign
Chapter 28 of the Holt Biology
Guided Audio CD Program to help
students achieve greater success in
reading the chapter. LS Auditory
Cnidocyte
Nerve
cell
Extracellular Digestion
In cnidarians and all subsequent animal phyla,
digestion begins extracellularly (outside the cell), in
the gastrovascular cavity. Enzymes break food
down into small fragments. Then cells lining the
cavity engulf the fragments, and digestion is completed intracellularly. This allows cnidarians to feed
on organisms larger than their own individual cells.
Teach
Tentacles
Nematocyst
(coiled)
Using the Figure
Mesoglea
Ask students to locate the gastrovascular cavity of the cnidarian in
Figure 7. Ask them how it compares
with that of the sponge. (The sponge
does not digest extracellularly in the
cavity, but within individual
choanocytes.) Point out the tissue
layers. Ask students how a tissue in
the cnidarians differs from the cells
in a sponge. (A tissue is a group of
cells with a common structure and
function.) LS Visual
Ectoderm
Gastrovascular
cavity
Endoderm
8
0
2
5
Estimating Size Using
a Microscope 2A 2C 6D
-7-0
TAKS 1 TAKS 2
Teaching Tip
You can use the microscope to estimate the size
of cnidarians that are too small to measure directly.
Materials
transparent millimeter ruler, compound microscope
with low-power objective or a dissecting microscope,
prepared slide of a medusa or polyp
Field of view
GENERAL
Graphic Organizer Refer students
to their graphic organizer from
Section 2. Ask the class to now fill
out the cnidarians’ row. LS Visual
Millimeter
marks
TAKS 2 Bio 10A
Analysis
Procedure
1. Identify the millimeter marks
along the edge of the ruler.
2. With the microscope on low
power (4⫻ or lower), place the
ruler on the stage and focus on
the millimeter marks.
3. Adjust the ruler so that one
edge lies across the diameter
of the field, as shown above.
Then measure the diameter of
the field of view in millimeters.
MATH TAKS 8, 8.9B; TAKS 10, 8.14A, C
4. Remove the ruler, and place
the prepared slide on the
stage. Identify the tentacles,
gastrovascular cavity, and
mouth.
1. Calculate the size of your
organism in millimeters by
multiplying the ratio you found
in step 5 by the width of the
field of view you found in step 3.
5. Estimate the length and width
of your organism as a ratio of
the width of the field of view.
For example, the length of your
organism may appear to cover
about two-thirds of the field
of view.
2. Describe the body plan of
the organism you viewed
using terms from step 4.
623
Graphic Organizer
Use this graphic organizer with Teaching Tip on
this page.
Animal groups
Cnidarians
Lifestyles
Free-floating
or attached
Structures/functions
Gastrovascular cavity/digestion
Tentacles/food getting
Cnidocytes/protection and food getting
18
493
x 2+ 6x
76
0
2
5
-7-0
<
x 2+ 6x
<
493
Estimating Size
TAKS 1 Bio/IPC 2A, 2C;
TAKS 3 Bio 6D (grade 10 only)
Skills Acquired
Collecting data,
measuring
Teacher’s Notes
Review with students
the proper procedures
for using a compound
microscope.
Answers to Analysis
1. Answers will vary, but should
be calculated by multiplying
the fraction of the field the
organism took up by the size
in mm measured across the
field of view.
2. Answers will vary. Refer to
Figure 6 for an illustration.
Transparencies
TT Cnidarian Body Plans
Chapter 28 • Simple Invertebrates
623
Hydrozoans
The most primitive cnidarians are members of class Hydrozoa.
Most species of hydrozoans are colonial marine organisms whose
life cycle includes both polyp and medusa stages. Freshwater hydrozoans are less common, but are familiar to many people because
they are often studied in school laboratories.
Teach, continued
continued
Using the Figure
Refer students to the illustration of
the tumbling hydra in Figure 8.
Tumbling may look like a difficult
way to move, so compare it to the
movement of a flexible toy spring
that can “walk” down a set of steps.
A hydra is similar in that once it
connects one end of its body to a
solid surface, the rest of its body
will flip over due to elasticity and
momentum. LS Visual TAKS 2 Bio 8C
Figure 8 Freshwater hydra
This tiny hydra is attached
to the leaf of a small aquatic
plant. One way a hydra can
move is by tumbling.
Freshwater Hydrozoa
Magnification: 34ⴛ
did you know?
Stolen Nematocysts Some sea
slugs (nudibranchs) feed on cnidarians and save the untriggered nematocysts within their own bodies.
When other animals try to eat the
sea slug, the stored-up nematocysts
are triggered. As a result of this
painful encounter, many predators
learn to avoid sea slugs.
Basal
disk
TAKS 3 Bio 12B
The abundant freshwater genus Hydra is
unique among hydrozoans because it
has no medusa stage and exists only as a
solitary polyp. Hydras live in quiet
ponds, lakes, and streams. They attach
to rocks or water plants by means of a
sticky secretion they produce in an area
of their body called the basal disk .
Hydras can glide around by decreasing
the stickiness of the material secreted by
their basal disk. Sometimes hydras move
by tumbling, as shown in Figure 8. To
tumble, the hydra bends its body over
and touches the surface it is attached to
with its tentacles. Then it pulls its basal
disk free, flipping it over to the other
side of its tentacles. The basal disk then
reattaches, and the hydra returns to an
upright position. Most hydras are brown
or white, like the one in Figure 8. Others
appear green because of the algae living
within their cells.
Marine Hydrozoa
Marine hydrozoans are typically far more complex than freshwater
hydrozoans. Often many individuals live together, forming colonies.
The cells of the colony lack the interdependence that characterizes
the cells of multicellular organisms. However, they often exhibit
considerable specialization. For example, the colonial Portuguese
man-of-war (genus Physalia) incorporates both medusas and
polyps. A gas-filled float (probably a highly modified polyp) allows
Physalia to float on the surface of the water. Dangling below the float
are tentacles that can reach 60 m (about 197 ft). These tentacles are
used to stun and entangle prey. Their nematocysts are tipped with
powerful neurotoxins (nerve poisons) that are dangerous and may
be fatal to humans. Physalia, shown in Figure 9, has other specialized polyps and medusas, each carrying out a different function,
such as feeding or sexual reproduction.
624
Cultural
Awareness
pp. 624–625
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 2 Bio 10A
TAKS Obj 3 Bio 7B
TAKS Obj 3 Bio 12B
TEKS Bio 7B, 8C, 10A, 12B
Teacher Edition
TAKS Obj 2 Bio 8C
TAKS Obj 3 Bio 7B, 12B
TAKS Obj 4 IPC 7A
TEKS Bio 7B, 8C, 12B
TEKS IPC 7A
624
A Mythological Hydra The name hydra is
a reference to the nine-headed serpent that
was slain by Hercules, a figure in Greek
mythology. According to Greek myth, the
slaying of the Hydra was one of the twelve
labors that Hercules was assigned to perform. Killing the Hydra was not an easy
task for Hercules because when one head of
the Hydra was cut off, two others would
grow in its place.
Chapter 28 • Simple Invertebrates
Reproduction in Hydrozoans
Most hydrozoans are colonial organisms whose polyps reproduce asexually by forming small buds on the body wall. The
buds develop into polyps that eventually separate from the
colony and begin living independently. Many hydrozoans are
also capable of sexual reproduction. Some species of Hydra are
hermaphrodites, but in most species the sexes are separate.
The genus Obelia is typical of many marine colonial hydrozoans. Obelia lives in colonies that form when one polyp asexually produces buds that do not separate from it. Eventually,
there are numerous polyps attached to one stem, forming the
colony. The Obelia colony shown in Figure 10 is branched like
deer antlers, with various polyps attached to the branched
stalks. The reproductive polyps give rise asexually to male and
female medusas. These medusas leave the polyps and grow to
maturity in the ocean waters.
During sexual reproduction, the medusas release sperm or
eggs into the water. The gametes fuse and produce zygotes that
develop into free-swimming, ciliated larvae called planulae
(PLAN yoo lee). The planulae eventually settle on the ocean bottom and develop into new polyps. Each polyp gives rise to a
new colony by asexual budding, and the life cycle is repeated.
Teaching Tip
Figure 9 Physalia. A single
Portuguese man-of-war colony can
contain 1,000 individual medusas
and polyps.
Magnification: 230ⴛ
In Obelia’s life cycle, the medusa stage (sexual) and the polyp
stage (asexual) alternate.
Immature medusa
Tentacles
GENERAL
Transparencies
Egg
Male medusa
Mouth
Using the Figure
Check for student understanding of
the life cycle of Obelia, shown in
Figure 10. Ask students the following questions: Which stages are
free-swimming and which are sessile? (medusa, polyp) Which stage
reproduces asexually? (polyp stage)
In which stage are there males and
females that reproduce sexually?
(medusa stage) LS Visual
Figure 10 Reproduction in Obelia
Reproductive
polyp
GENERAL
Planulae There are both negative
and positive aspects to the existence of the free-swimming planulae of Obelia. Ask students about
the dangers to free-swimming larvae in the ocean, and why it is an
advantage for larvae to be able to
swim far from their parents.
(Larvae may starve, be eaten, wash
up on shore, or not find a suitable
place to settle down. Motile larvae
allow Obelia to spread to new
places.) LS Verbal TAKS 3 Bio 7B
Female medusa
TT Reproduction in Obelia
Sperm
Early embryo
Obelia colony
Planula
625
IPC Benchmark Review
To prepare students for the TAKS and accompany
the discussion of cnidarians, have students review
Buoyancy, TAKS 4 IPC 7A on p. 1049 of the IPC
Refresher in the Texas Assessment Appendix of
this book.
MISCONCEPTION
ALERT
The Portuguese Man-of-War Many people think that the Portuguese man-of-war
(Physalia) is a jellyfish because it is shaped
somewhat like one. However, the term jellyfish is used for scyphozoans, a class of
cnidarians in which the medusa form dominates the life cycle. Physalia is actually a
hydrozoan and exists as a colony of
medusas and polyps. TAKS 3 Bio 7B
Chapter 28 • Simple Invertebrates
625
Teach, continued
continued
READING
SKILL
BUILDER
Figure 11 Marine jellyfish
Aurelia. Aurelia polyps are
about the size of hydras. The
free-swimming medusas
range from 10 cm (3.9 in.) to
25 cm (9.8 in.) in diameter.
GENERAL
Discussion Jellyfish have toxins
in their nematocysts that paralyze
and kill fish. Ask students why it is
important for a simple floating animal like a jellyfish to immobilize its
prey. (A jellyfish cannot stalk or
chase prey, and it has no jaws or
claws to capture prey.) LS Logical
Polyps
Scyphozoans
TAKS 3 Bio 7B, 12B
Using the Figure
GENERAL
Have students study Figure 11 to
compare and contrast the medusa
and polyp stages of a jellyfish. Ask
students to name other animals
that have different bodies during
different stages of their life. (hydrozoans, insects, frogs, and so on) Then
ask students to compare Aurelia
medusae and polyps to those of
Obelia. (They are almost identical.)
Ask students to identify the big
difference in the life cycles of
hydrozoans and scyphozoans.
(Hydrozoans spend most of their life
as a polyp; scyphozoans spend most
of their life as a medusa.) LS Visual
Medusa
The name Scyphozoa is
from the Greek skyphos,
meaning “cup,” and zoia,
meaning “animal.” The
name refers to the fact that
members of this class
spend most of their lives as
medusas, which have the
shape of an inverted cup.
Cnidarians belonging to the class Scyphozoa (sie fuh ZOH uh) are
the organisms usually referred to as true jellyfish. Scyphozoans are
active predators that ensnare and sting prey with their tentacles.
The toxins contained within the nematocysts of some species are
extremely potent. Scyphozoans range in size from as small as a
thimble to as large as a queen-size mattress.
The jellyfish seen in the ocean are medusas, which reproduce sexually. However, most species of jellyfish also go through an inconspicuous polyp stage at some point in their life cycle. The stinging
nettle, Aurelia, shown in Figure 11, is one of the most familiar jellyfishes. Aurelia’s tiny polyps hang from rocky surfaces. Periodically
the polyps release young medusas into the water. The Aurelia life
cycle is similar to that of Obelia, pictured on the previous page. The
major difference is that Aurelia spends most of its life as a medusa,
while Obelia spends most of its life as a polyp.
Jellyfish Relatives
Related to the jellyfish are the cubozoans, or box jellies. As their
name implies, cubozoans have a cube-shaped medusa. Their polyp
stage is inconspicuous, and in some species, it has never been
observed. Most box jellies are only a few centimeters in height,
although some are 25 cm (10 in.) tall. A tentacle or group of tentacles is found at each corner of the “box.” Stings of some species,
such as the sea wasp, can inflict severe pain and even death among
humans. The sea wasp lives in the ocean along the tropical northern
coast of Australia.
Other relatives are members of the phylum Ctenophora (tehn AW
for uh), which includes the comb jellies. Comb jellies differ from
true jellyfish in two major ways—they have only a medusa stage and
they have no cnidocytes. Their tentacles are covered with a sticky
substance that traps plankton, the comb jelly’s main prey. Although
a comb jelly is only about 2.5 cm (1 in.) in diameter, its tentacles can
be 10 times as long.
TAKS 2 Bio 6D (grade 10 only)
626
Cultural
Awareness
pp. 626–627
Student Edition
TAKS Obj 2 Bio 8C
TAKS Obj 3 Bio 7B
TEKS Bio 7B, 8C
Teacher Edition
TAKS Obj 2 Bio 6D, 8C
TAKS Obj 3 Bio 7B, 12B
TAKS Obj 4 IPC 7A, 7D
TEKS Bio 6D, 7B, 8C, 12B
TEKS IPC 7A, 7D
626
Edible Jellyfish In many Asian countries,
some species of jellyfish are considered delicacies. Their taste has been compared with
that of pickles. Although jellyfish have
stinging cells, the salting process used to
prepare them as food breaks down the toxins in these cells.
Chapter 28 • Simple Invertebrates
IPC Benchmark Mini-Lesson
Biology/IPC Skills TAKS 4 IPC 7A
Investigate properties of fluids, including buoyancy.
Activity Fluids exert an upward force on objects
called buoyant force. This force is the reason objects
feel lighter in water than they do in air. According to
Archimede's principle, the buoyant force exerted on
an object is equal to the weight of the fluid that is
displaced by the object. If an object weighs more
than the fluid it displaces, it will sink. Ask students
why jellyfish float, why rocks sink, and what a person
might be able to do to make a rock float. (Jellyfish
are lighter (less dense) than the water that would be
displaced by their bodies, rocks weigh more (are
more dense), but could be made to float by placing
them on a platform that is less dense than water.)
Anthozoans
The largest class of cnidarians is class Anthozoa. Anthozoans exist
only as polyps. The most familiar anthozoans are the brightly colored
sea anemones and corals. Other members of this class are known by
such fanciful names as sea pansies, sea fans, and sea whips.
Anthozoans, such as the sea anemone shown in Figure 12, typically have a thick, stalklike body topped by a crown of tentacles that
usually occur in groups of six. Nearly all of the shallow-water
species contain symbiotic algae, such as dinoflagellates. The anthozoans provide a place for the dinoflagellates to live in exchange for
some of the food that the dinoflagellates produce. The brilliant color
of most anthozoans is actually that of dinoflagellates living within it.
Some anthozoans reproduce asexually by forming buds, but they
also reproduce sexually by releasing eggs and sperm into the ocean,
where fertilization occurs. The fertilized eggs develop into planulae
that settle and develop into polyps.
SKILL
Figure 12 Sea anemone.
When threatened, the sea
anemone quickly retracts its
tentacles and compresses
its body.
Sea Anemones
Sea anemones are a large group of soft-bodied polyps found in
coastal areas all over the world. Many species are quite colorful, and
most do not grow very large, only from 5 mm (0.2 in.) to 100 mm
(4.0 in.) in diameter. Sea anemones feed on fish and other marine
life that happen to swim within reach of their tentacles.
Sea anemones are highly muscular and relatively complex animals. When touched, most sea anemones retract their tentacles into
their body cavity and contract into a tight ball. Sea anemones often
reproduce asexually by slowly pulling themselves into two halves.
This method of reproduction often results in large populations of
genetically identical sea anemones.
Corals
Most coral polyps live in colonies called reefs, such as the one
shown in Figure 13. Each polyp secretes a tough, stonelike outer
skeleton of calcium carbonate that is cemented to the skeletons of
its neighbors. (Some corals called soft corals do not secrete hard
exoskeletons.) Only the top layer of a coral reef contains living coral
polyps. When coral polyps die, their skeletons remain and provide a
BUILDER
Writing Skills Have students
research the relationship between
some species of sea anemones and
certain types of fishes, such as the
damselfish and the clownfish. The
clownfish gets protection by hiding
in the tentacles, and the sea
anemone gets cleaned as the
clownfish eats undigested debris
and parasites. Students should
investigate how these fishes are
able to avoid discharging the
anemone’s nematocysts. Ask
students to write a brief report.
LS Verbal TAKS 3 Bio 12B
Demonstration
Figure 13 Coral. This coral
reef is made up of hundreds of
thousands of individual coral
polyps. When the polyps feed
(inset), they extend their tentacles from the protection of their
stony skeleton.
Collect photographs or specimens
of coral for students to observe.
Try to have some photos showing
coral polyps. Some students find it
difficult to believe that cnidarians,
such as sea anemones and corals,
are animals. Ask students to defend
the classification of anthozoans in
the animal kingdom. LS Visual
TAKS 2 Bio 8C
Using the Figure
GENERAL
Have students look closely at the
insert of the coral polyp shown in
Figure 13. Ask how the polyp
resembles the polyp form of other
cnidarians. (It is cup-shaped and
contains a ring of upward-facing
tentacles surrounding the opening
to its exterior.) LS Visual
TAKS 2 Bio 8C
627
IPC Benchmark Fact
Have students identify the elements found in the compound calcium carbonate. Then, ask them to explain
the bonding relationships between the elements
based on their respective locations on the periodic
table. TAKS 4 IPC 7D (grade 11 only)
Chapter 28 • Simple Invertebrates
627
foundation for new coral polyps. Over thousands of years, these formations build up into coral reefs where hundreds of thousands of
polyps live together on top of old skeletons. Coral reefs are found
primarily in tropical regions of the world, where the ocean water is
warm and clear, an environment that is ideal for the corals and the
dinoflagellates that live inside them.
Teach, continued
continued
Texas’s Inland Reef
TAKS 3
Bio 7B
Have students look at the photograph of El Capitan in this
feature and identify the fossil
reef and remaining sediment
bands.
Discussion
Ask students how El Capitan
was formed. Ask them if the
bands would have fossil organisms. (The bands are composed
of sediment, with the reef forming from sponges, algae and bryozoans. The overlying sediments
deposits have been eroded away.)
Close
Reteaching
Have students make a table that
shows the similarities and differences between the three classes of
cnidarians: Hydrozoa, Scyphozoa,
and Anthozoa. (Similarities: radial
symmetry, aquatic lifestyle, presence
of cnidocytes. Differences: predominance of a polyp or medusa body
form, the maintenance of a solitary
versus colonial lifestyle.) LS Verbal
TAKS 2 Bio 6D (grade 10 only); Bio 8C
Texas’s Inland Reef
O
ne of the finest examples
of an ancient fossil reef
can be found in the Guadalupe
Mountains of western Texas.
This mountain range includes
El Capitan, a massive limestone
cliff, and Guadalupe Peak, the
highest point in Texas at 2,667 m
(8,749 ft). The Guadalupe Mountains comprise part of the
Capitan Reef, which stretches
from southeastern New Mexico
as far south as Alpine, Texas.
History of the Capitan Reef
The Capitan Reef formed about
250 million years ago. At that
time, the area that is now western
Texas and southeastern New
Mexico was covered by an inland
sea, which was connected to the
ocean by a narrow inlet. The area
was located much closer to the
equator than it is today. The warm
inland sea hosted a rich diversity
of organisms, including horn
corals, sea urchins, flowerlike
crinoids, clamlike brachiopods,
and trilobites. Over several million
years, the Capitan Reef formed as
invertebrate skeletons accumulated and were cemented in place
by encrusting organisms. Unlike
modern reefs, which are built
mainly by corals, the Capitan
Reef was constructed by algae,
sponges, and tiny colonial animals called bryozoans.
Eventually, the inland sea
became separated from the ocean
and began to dry up. Minerals precipitated out of the sea, forming
thin bands of sediments that covered the reef over thousands of
years. Around the same time,
GENERAL
1. List the names and location of
the two layers of tissues found in
the cnidarians. (ectoderm on the
outer surface, endoderm on the
inner surface)
2. Describe the digestive process of
the cnidarians. (Digestion occurs
extracellularly inside the gastrovascular cavity.)
pp. 628–629
Student Edition
TAKS Obj 2 Bio 10A
TAKS Obj 2 Bio 8C
TAKS Obj 3 Bio 7B
TEKS Bio 3E, 7B, 8B, 8C, 10A, 11B
Teacher Edition
TAKS Obj 2 Bio 6D, 8C, 10A
TAKS Obj 3 Bio 7B
TEKS Bio 3E, 6D, 7B, 8B, 8C, 10A, 11B
628
El Capitan
a great mass extinction occurred.
Horn corals, trilobites, and many
groups of sponges and algae
died out, including most of the
reef builders.
For 200 million years, the
Capitan Reef lay buried under
sediments. Then, about 26 million years ago, faulting lifted the
reef 3 km above its original position. Wind and rain eroded the
overlying sediments, revealing
the reef once again.
Section 2 Review
Compare the two body forms of cnidarians.
Critical Thinking Forming Hypotheses
8C
Some cnidarians are unique in exhibiting polyp
and medusa forms. How might their two body
forms give them an advantage over species that
live in the same environment but have only one
7B 8C
body form?
Relate cnidocytes and nematocysts to food
gathering.
10A 11B
Draw and label the life cycle of Obelia.
Quiz
TAKS 3
3E 8B 8C
Summarize the similarities and differences in
the three classes of cnidarians described.
8B 8C
TAKS Test Prep In cnidarians, digestion
takes place
10A
A only extracellularly. C in a gastrovascular
cavity.
B only intracellularly. D in a digestive tract.
Distinguish between the two types of asexual
reproduction found in cnidarians.
10A
628
Answers to Section Review
1. A medusa is often umbrella shaped. A polyp is
tube-like and closed at one end. TAKS 2 Bio 8C
2. Cnidocytes are stinging cells used in feeding. They
contain a nematocyst, which when triggered, is
shot into the cnidarian’s prey. TAKS 2 Bio 10A; Bio 11B
3. Drawing should be similar to Figure 10.
TAKS 2 Bio 8C; Bio 8B; Bio 3E
4. All cnidarians have stinging cells on their tentacles
and two cell layers—a middle layer of mesoglea,
and a gastrovascular cavity. Hydrozoans are primarily colonial and spend most of their life as a
polyp. Scyphozoans exist primarily as a medusa.
Anthozoans are only polyps and may be solitary
or colonial. TAKS 2 Bio 8C; Bio 8B
Chapter 28 • Simple Invertebrates
5. A colonial cnidarian may reproduce by budding. Some may reproduce asexually by slowly
pulling themselves in half. TAKS 2 Bio 10A
6. The two body forms exploit different environments, so these cnidarians have access to a
broader array of resources while spreading
their risks between two lifestyles.
TAKS 2 Bio 8C; TAKS 3 Bio 7B
7.
A. Incorrect. Digestion occurs
extracellularly. B. Incorrect. Digestion occurs
intracellularly after the cells of the cavity engulf
nutrients. C. Correct. Digestion occurs in a
gastrovascular cavity. D. Incorrect. Cnidarians
do not have a digestive tract. TAKS 2 Bio 10A